1. Field of the Invention
The present invention relates to a printed wiring board on which high-density wiring or electronic components can be mounted. In addition, the present invention relates to a method of manufacturing a printed wiring board with a high yield while reducing the number of processes.
2. Description of the Related Art
Double-sided and multilayer printed wiring boards comprise various wiring patterns. For example, each face of the double-sided printed wiring board comprises wiring patterns such as double-sided conductive patterns. Likewise, wiring patterns are formed layer by layer of the multilayer printed wiring boards. The wiring patterns of one face or layer are electrically connected to those of the other(s). In this event, the connections between the wiring patterns can be achieved in a following manner.
For the double-sided printed wiring board, a base plate is first laminated with a copper foil on both sides thereof. A number of through-holes are formed through the base plate at predetermined positions thereon. Subsequently, an entire surface, including the inside of the through-holes, is plated by a chemical plating process. A conductor layer is then deposited on the inner walls of the through-holes by an electroplating process to increase a thickness of the conductor (metal) layer. This electrically connects the wiring patterns of one face to those of the other face with a higher reliability.
On the other hand, for the multilayer printed wiring board, a double-sided wiring board is formed by means of patterning copper foils laminated on both sides of a base plate. Copper foils are positioned and disposed on the patterned surfaces through an insulating sheet (e.g., prepreg sheets). The resultant structure is then integrally bonded under heat and pressure. Subsequently, as in the case of the above mentioned double-sided printed wiring board, through-holes are formed and the wiring patterns of the individual layers are electrically connected to each other by a plating process. Further, the copper foils positioned and disposed on the patterned surfaces are patterned to obtain a multilayer wiring board. A multilayer printed wiring board having many patterned layers may be manufactured through a method with an increased number of interposed double-sided printed wiring boards.
Alternatively, printed wiring boards may be manufactured according to a method in which the wiring patterns are connected without using a plating process. In this method, a number of through-holes are formed through the base plate at predetermined positions thereon. A conductive paste having a resin component is embedded in the through-holes, for example, by a printing technique. The resin component of the conductive paste is then solidified or cured in the through-hole to connect the wiring patterns electrically.
For the above mentioned printed wiring board and the method of manufacturing it by using a plating process for the electrical connections between the wiring patterns, the manufacturing process requires steps of forming the through-holes and of plating the inside of the through-holes for the electrical connections. This disadvantageously makes the manufacturing process for the printed wiring board become excessive, resulting in the necessity for complicated process control. The step of forming the through-holes is required even for the case where the electrical connection is achieved by means of embedding the conductive paste in the through-holes, as in the case where it is achieved by the plating. It is difficult to embed the conductive paste uniformly in the through-holes, and there is some question about long-term reliability of the electrical connections. In any case, the need for forming the through-holes to connect the wiring patterns electrically affects cost and yield for the resulting printed wiring board, which may oppose a current cost-saving demand. Further, the through-holes are opened on both sides of the printed wiring board. No wiring pattern can be formed on the opened through-holes. Likewise, no through-hole can mount electronic components thereon. Accordingly, the through-holes limit the possible improvement of a wiring density as well as of a packaging density for the electronic components on the printed wiring board.
The printed wiring board may be imparted with a heat sinking property to stabilize operation of a circuit device formed on the printed wiring board. The printed wiring board may also be imparted with grounding and shielding properties to avoid damage of a circuit and to suppress radio frequency noise, respectively. For this purpose, the printed wiring board comprises a thin metal plate as a base plate (hereinafter, referred to as a metal-base type printed wiring board). Alternatively, the printed wiring board comprises a thin metal plate as a core (hereinafter, referred to as a metal-core type printed wiring board). The thin metal plate serves as a support, a reinforcement and a heat sinking element as well as a ground electrode.
The printed wiring boards of the metal-base and metal-core types still have some disadvantages. As in the above mentioned case, the through-holes are required to be formed by means of drilling a plate for lamination or a laminated plate. The plating layer is deposited on the inside of the through-holes and a conductive metal or a conductive composition is embedded therein. The electrical and thermal conductivities of the printed wiring board are thus provided by the conductive metal or composition as well as the plating layer. Such printed wiring boards can only be manufactured through complicated steps. For the printed wiring board with the interconnections between the wiring patterns achieved by the through-holes, the area available for wiring and packaging on the surface is reduced due to these through-holes. There is thus a limit to the degree of densification of the high-density package circuit with the printed wiring board of the type described.
Some printed wiring boards also have sealing properties. For this purpose, a copper paste or the like is applied to a substrate. The wiring patterns are arranged on a substrate and the copper paste or the like is applied to an area of the wiring patterns through an insulating layer to form a desired shielding layer. The shielding layer is connected to a ground electrode (ground layer) on the substrate and covered with another insulating layer.
The printed wiring board of the type described has problems of insufficient shielding effects and stabilities. Probable reasons for such problems are as follows: the copper paste typically has a high conductor resistance and it is difficult to control the thickness of the coated layer uniformly. In addition, the area to which the shielding properties are imparted projects to a higher position than the remaining portions, so that packaging conditions for a packaging machine should be controlled and modified depending on demand during a subsequent process of forming a package circuit device.
The conventional printed wiring boards are insufficient for complying with the demands on minimizing the circuit devices by the high-density wiring and packaging. There have been strong demands on more effective, cost-saving, and practical printed wiring boards and a method of manufacturing the same.
Accordingly, an object of the present invention is to provide a printed wiring board having high reliability having a simple structure.
Another object of the present invention is to provide a printed wiring board having a simple structure, on which high-density wiring and electronic components can be mounted.
It is another object of the present invention to provide a printed wiring board capable of being manufactured through a simple process, on which high-density wiring and electronic components can be mounted.
It is yet another object of the present invention to provide a method of manufacturing printed wiring boards on which high-density wiring and electronic components can be mounted.
It is still another object of the present invention to provide a method of manufacturing printed wiring boards of high reliability.
Yet another object of the present invention is to provide a method of manufacturing printed wiring boards of high quality with a high yield.